Upon incubation with pharmacologically relevant concentrations of carrageenan (100 Mug/m1), isolated pleural macrophages previously labeled with (14C)arachidonic acid released 2.5 times the radioactivity of control cells. The main components of the released radioactive materials were arachidonic acid and, to a much lesser extent, PGE2 and leukotriene C4, both of which are known to be vasoactive. In the absence of carrageenan, indomethacin (20 Mum) augmented the release of arachidonic acid but suppressed the formation of PGE2, implying that the cyclooxygenase in resting pleural macrophages is capable of converting the released arachidonic acid to PGE2. That carrageenan can stimulate release of arachidonic acid from pleural macrophages indicates that activation of phospholipase A2 is involved in the carcageenan action and that PGE2 and LTC4 may mediate the inflammatory response to carrageenan in vivo. Furthermore, our findings suggest that the macrophages may play a role in the initiation of the early event in the inflammatory response.